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Volcanic hazards posed by the dynamic combination of water, sediment, and gravity can be persistent, costly, and deadly. Monitoring hydrologic hazards helps USGS scientists understand and help mitigate the risks due to destructive lahars and excess sediment in volcanic river channels.
Many young volcanoes are covered with sparsely vegetated deposits consisting of fine ash and loose, fragmented rocks. In these landscapes rain, that elsewhere would fall on vegetation and soak into mature soil, instead falls directly on the ground and flows freely overland. This water easily erodes volcanic ash and rock debris from hill slopes quickly carving broad channels and transporting enormous quantities of sediment to areas tens of kilometers downstream. This excess sediment can fill channels farther away from the volcano, resulting in raised riverbeds, impounded lakes, and more frequent flooding. Water flowing vigorously in these settings has the force to cut through lake impoundments to release lahar-generating floods.
Since the 1980 Mount St. Helens eruption, rivers leading from the volcano have acted as redistribution channels for large volumes of volcanic sediment. The Toutle River consistently transports up to 100 times more sediment than it did prior to the eruption even though erosion in the upper valley has decreased two-fold since the late 1980's. As a long-term strategy to remediate excess sedimentation and prevent flooding, a sediment retention structure was built across the North Fork Toutle River in 1987 to trap sediment before it could be carried downstream to the Cowlitz and Columbia Rivers. The lower Cowlitz River is subject to riverbed aggradation (increased sediment accumulation), which puts several communities at increased flood risk. The Columbia River is a primary U.S. shipping channel and sediment continues to be a significant economic burden to industry.